Known from Swiss Pat. Nos. 497,103, 550,521 and 554,114 are digitally-working loop systems in which the transfer of information between the connected subscriber stations is effected by code-addressed telegrams. Such systems are used generally for the exchange of information in fairly closely limited, local regions.
Furthermore known nowadays is the PCM technique for the multiple use of lines. It is used particularly for the connection of telephone exchanges with one another. With the IFS project of the Swiss PTT an attempt is being made to use the PCM-technique in the wider framework of the communications technique. For example Swiss Pat. No. 550,519 gives information on this. The PCM technique ends here too in telephone exchanges.
Known through "Bell Laboratories Record", volume 50, No. 3, March 1972, pages 80-86, there is finally a conventionally-working exchange network to which a PCM loop system is connected. The transition from the analogue technique to the digital technique is effected by Delta modulation and by the coordination of time slots to the analogue lines which are selected via a conventional exchange apparatus as well as to the subscriber stations. In this way the exchange of information between these stations and the network is possible.
The task of the present invention now consists in operating, in PCM networks, the digital technique to the subscriber stations.